Could be called a game project, though. If it does, the Freespace 2 Source Code Project began active development around the same time as Morrowind (Freespace 2 source code released in 2002, first SCP release in June 2002). Still, even with such a broad interpretation TR is definitely one of the oldest projects still ongoing.

The ZDoom team probably wasn't active the whole time through but that is indeed an impressive development time!

Finding out exactly when proper TR planning began was a bit... tricky, so I defaulted in favour of when the TESCS was released and allowed planning that took the TESCS' possibilities and limitations into account.

The project was created in late 2001 before the release of the game when Bethesda announced that the game would only include Vvardenfell and not the entire province of Morrowind. Naturally, the work done before the release was almost entirely preparatory. The project originally planned to create all of Tamriel in the game (hence the project's name) until as late as 2005.